This invention relates to a device for use in generation of ultrasonic signals. More particularly, the invention relates to a transducer employing a piezoelectric crystal for generating ultrasonic signals for use in through transmission ultrasonic (TTU) inspection.
TTU inspection procedures are widely used in non-destructive inspection applications such as inspection of laminated structural components. To undertake TTU inspection, a transducer generating an ultrasonic signal is aligned with an ultrasonic signal-receiving transducer, and a workpiece to be inspected is interposed between the transducers. Although the signal may be transmitted directly into an adjacent workpiece, a liquid coupling medium such as a stream of water is generally provided to allow passage of ultrasonic energy from the generating transducer to the workpiece and from the workpiece to the receiving transducer. A generating transducer and a receiving transducer may form a pair which act as a unit when they are aligned with each other on opposite sides of the workpiece being inspected.
TTU inspection is often conducted with an array of pairs of generating and receiving transducers arranged in a line. The array of transducer pairs with associated streams of coupling medium is passed over the workpiece being inspected so as to come into contact with all portions of the workpiece to produce an output signal which varies with the thickness of the workpiece and with the acoustic attenuation of the material forming the workpiece.
The output signal, representative of the ultrasonic signal intensity received at each point along the workpiece, may than be recorded on an X-Y display, with the brightness or density at each point on the display correlating to the intensity of the ultrasonic signal received at the corresponding point on the workpiece. Light shaded areas on the display indicate workpiece areas of lowered ultrasonic signal transmitting characteristics. If the low-intensity signal transmission is not due to extra thickness of the workpiece or to a known change in workpiece material composition, the low intensity shown on the X-Y display is probably indicative of a defect in the laminate layers or their bonding.
The inspection procedure described above requires precise alignment between the liquid coupling medium of the ultrasonic signal generating transducer and the liquid coupling medium of the ultrasonic signal receiving transducer. The streams of liquid that comprise the coupling media must contact the workpiece being inspected at points along the central axis of the transmitting and receiving transducers on opposite sides of the workpiece. For a conventional transducer generating signals at 1 MHz, the liquid streams must both contact the substrate within .+-. 0.10 inches of the generator/receiver central axis.
When a transducer generates signals for TTU inspection, an ultrasonic signal with a frequency of 1 MHz is often used. Such signals can transfer large amounts of ultrasonic energy into the workpiece and thus provide excellent results when testing relatively thick workpieces and workpieces constructed of material having high ultrasonic attenuation, such as honeycomb material.
When thin laminated components are inspected with a conventional 1 MHz ultrasonic signal, only limited attenuation occurs when the signal passes through a defective portion of the workpiece. This yields a low-contrast display in which a defect is often difficult to detect. Greater signal attenuation may be achieved with thin laminated workpieces for improved defect detection if the ultrasonic signal is generated at a frequency of 5 MHz. Unfortunately, the beam pattern of a 5 MHz frequency signal generated by prior art transducers is much less consistent than the beam pattern of a signal with a 1 MHz frequency generated by the same prior art transducers. The 5 MHz ultrasonic signal beam patterns of prior art transducers are such that signals of near peak amplitude are generated over only a narrow area at the center of the transducer. Outside of this small center area, or main lobe, the amplitude of the generated signal fluctuates irregularly.
Pairs of prior art transducers operating at 5 MHz must be positioned such that streams of coupling media are aligned to within .+-. 0.03 inches. However, the force of gravity on such streams makes such alignment extremely difficult, and restricts the ability to generate accurate TTU data for detecting bonding defects in a thin laminated workpiece.
If the higher ultrasonic signal frequency is not used, the maximum signal strength is consistent over a greater area. However, bonding defects in thin laminates cannot be readily detected because poorly bonded areas do not significantly attenuate a 1 MHz ultrasonic signal. It is therefore an object of the present invention to provide an ultrasonic transducer whose signal is substantially attenuated by defects in a thin workpiece and to shape the beam intensity profile of, or apodize, the transducer to permit less critical alignment of coupling media. It is a further object of the invention to provide a transducer suitable for use with workpieces having a wider range of thickness and material composition.